1. Field of the Invention
This invention relates generally to measuring and testing and more particularly to measuring torsional and bending stress of a drill bit during a drilling operation.
2. Description of the Prior Art
Circuit boards generally comprise a multi-layered substrate formed of a synthetic material such as glass-filled epoxy. Each layer includes metallic circuit connections formed thereon. Integrated circuit chips are mounted on the boards by inserting pins through via holes formed in the board. The via holes provide circuit interconnections between the chips and the various layers of the board as is well known. After the holes are drilled, solder plating is applied through each hole to provide electrical interconnection between the layers.
The via holes are very small and range upward from 5 mils. Accuracy, within a fraction of a mil, is important in forming the via holes. The via holes are formed by drilling so the drill must be made with great precision. These drills are commonly operated at speeds of up to 100,000 rpm and drill from about 50 to about 60 holes per minute.
Not only must these drills be made with precision but they must be used with precision. If drilling is improper, the quality of the via holes is unacceptable in terms of hole geometry, smear, electrical integrity and long term stability. Also, improper use will shorten drill life. Thus, it is important to monitor the drilling operation to assure that it is conducted within proper limits.
During a drilling operation, the via holes are drilled at the rate of about 1 hole per second. The drill passes through the board at about 3-4 mils per revolution. The board material offers resistance to the drill which results in torsional and bending stress acting on the drill. Torsional stress is caused by friction between the board and drill. The friction generates heat. Bending stress is caused by axially forcing the drill through the board. With the advent of other board materials, such as ceramics, drill resistance will be increased.
Various drilling monitor operations used in the past have included accelerometers used for vibration signature analysis, infra-red (IR) to monitor heat generation, and acoustic sensors to detect noise variations. These operations are limited; they monitor secondary parameters such as vibration, noise and heat rather than drill stress, a primary parameter, so that readings are modified and are not precise. Also, response time is slow. For example, the readings produced by accelerometers and IR monitors are modified by the media being drilled, and results produced by acoustic monitors are modified by the media and by the surrounding air.
The foregoing limitations vary, of course, with material to be drilled, the drill design and the material of the drill; there are many variables. Thus there is a need for a drilling monitor having fast response time, precision, and which monitors the drill stress to produce an unmodified signal.
The foregoing illustrates limitations of the known prior art. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations as set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.